1. Field of the Invention
This invention relates to polyester multifilament yarn with high modulus and low shrinkage particularly useful for the textile reinforcement of tires. The yarn of the invention provides high treated cord tenacity while maintaining or increasing treated cord dimensional stability when compared to prior art yarns. A process for,production of the multifilament polyester yarn is an aspect of the invention.
2. Description of the Prior Art
Polyethylene terephthalate filaments of high strength are well known in the art and are commonly utilized in industrial applications including tire cord for rubber reinforcement, conveyor belts, seat belts, V-belts and hosing.
Continued improvement in high strength industrial yarns particularly suited for use as fibrous reinforcement in rubber tires is an ongoing need in the industry. In particular, the improvement of treated cord tenacity and dimensional stability are desired objectives. U.S. Pat. No. 4,101,525 to Davis et al. provides an industrial strength multifilament polyester earn with high initial modulus and low shrinkage. Although Davis et al. does not provide treated cord data, it is commonly known that compared to conventional tire cords such yarn provides a reduced tenacity when the yarn is converted to the treated tire cord. Additionally, rapid cooling of the filament immediately after emerging from the spinneret can result in excessive filament breakage and thus yield yarn with poor mechanical quality. U.S. Pat. No. 4,491,657 to Saito et al. discloses high modulus, low shrinkage polyester yarn, but requires a low terminal modulus to achieve good yarn to treated cord conversion efficiency for such dimensionally stable yarns. The low terminal modulus is carried over into the treated cord and results in a lower tenacity than the high terminal modulus cords of the present invention. Also, as shown in FIG. 8, the process of Saito et al. requires high spinning speeds, which makes it difficult to process on-panel, i.e. a continuous spin-draw process.
Polyethylene terephthaleate yarn can be prepared by spinning under high stress conditions in the transition region between oriented-amorphous and oriented-crystalline undrawn yarns. The invention is accomplished by selection of process parameters to form an undrawn yarn that is a crystalline, partially oriented yarn with a crystallinity of 3 to 15 percent and a melting point elevation of 2 to 10xc2x0 C. The spun yarn is then hot drawn to a total draw ratio between 1.5/1 and 2.5/1 with the resulting unique combination of properties: (A) a terminal modulus of at least 20 g/d, (B) a dimensional stability defined by E4.5+FS less than 13.5 percent, (C) a tenacity of at least 7 grams per denier, (D) a melting point elevation of 9 to 14xc2x0 C., and (E) an amorphous orientation function of less than 0.75. The drawn yarn is twisted and plied to produce tire-cord and then treated with resorcinol-formaldehyde-latex. The resulting treated tire cord unexpectedly provides high tenacity in combination with improved dimensional stability.